The present invention relates generally to artificial cardiac pacemakers, and more particularly to an implantable bradycardia pacemaker having switched capacitor amplifier circuits for various functions, including sensing of cardiac activity, providing a voltage reference in conjunction with such sensing, and voltage regulation.
In the normal human heart, the sinoartrial (S-A) node is the primary natural pacemaker by which rhythmic electrical excitation is developed. The cardiac impulse generated at the S-A node is transmitted to the atrial chambers at the right and left sides of the heart. In response, the atria contract, pumping blood from those chambers into the respective ventricular chambers. The impulse is transmitted to the ventricles through the atrioventricular (A-V) node, which imposes a delay, and via a conduction system comprising the bundle of His, the right and left bundle branches, and the Purkinje fibers. In response, the ventricles contract, the right ventricle pumping unoxygenated blood through the pulmonary artery to the lungs and the left ventricle pumping oxygenated (arterial) blood through the aorta and the lesser arteries to the body.
The right atrium receives the venous (unoxygenated) blood from the upper part of the body (head, neck and chest) via the superior vena cava, or upper great vein, and from the lower part of the body (abdomen and legs) via the inferior vena cava, or lower great vein. The blood oxygenated by the lungs is carried via the pulmonary veins to the left atrium.
This action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill. One-way valves along the veins, between the atrial and ventricular chambers in the right and left sides of the heart (the tricuspid valve and the mitral valve, respectively), and at the exits of the right and left ventricles (the pulmonary and aortic valves, respectively) prevent backflow of the blood as it moves through the heart and the circulatory system.
The S-A node is spontaneously rhythmic, and the normal cardiac rhythm originating therefrom is termed sinus rhythm. Disruption of the natural pacemaking and propagation system occurs as a result of aging or disease, and is commonly treated by artificial cardiac pacing. The artificial pacemaker is implanted to deliver rhythmic electrical to the heart as necessary to effect stimulation at the desired rate. Bradycardia pacers are designed to sense cardiac activity at a rate lower than the normal sinus rate range, and to return the rate to a selected value within that range. In its simplest form, the pacemaker consists of a pulse generator powered by a self-contained battery pack, and a lead including at least one stimulating electrode electrically connected to the pulse generator. The lead is typically of the catheter type for intravenous insertion to position the stimulating electrode for delivery of electrical impulses to excitable myocardial tissue in the appropriate chamber at the right side of the patient's heart. Usually, the pulse generator is surgically implanted in a subcutaneous pouch in the patient's chest. In operation, the electrical stimuli are delivered to the excitable cardiac tissue via an electrical circuit that includes the stimulating and reference electrodes and the body tissue and fluids.
Typically, the pacemaker is designed to operate in one of three different response modes, namely, asynchronous (fixed rate), inhibited (stimulus generated in absence of specified cardiac activity), or triggered (stimulus delivered in response to specified activity). The demand ventricular pacemaker has been the most widely used type, sensing the patient's natural heart rate and applying stimuli only during periods when the rate falls below the preset pacing rate.
Pacemakers range from the simple fixed rate device that provides pacing with no sensing function, to the highly complex model implemented to provide fully automatic dual chamber pacing and sensing functions. The latter type of pacemaker is the latest in a progression toward physiologic pacing, that is, the mode of artificial pacing that restores cardiac function as much as possible toward natural pacing.
In copending U.S. patent application Ser. No. 07/203/322 of Baker et al., titled "Implantable Cardiac Stimulator with Automatic Gain Control and Bandpass Filtering in Feedback Loop", assigned to the same assignee as is the present application, a cardiac stimulator is disclosed in which the electrical signal representative of detected cardiac activity is subjected to automatic gain control and bandpass filtering. The resulting signal is processed for comparison with inner and outer targets, or reference levels, to determine of the nature of the cardiac activity and ultimately to correct abnormalities in that activity. The device described in the Baker et al. application is primarily concerned with tracking rapidly varying signals of the type commonly associated with fibrillation, in which the heart undergoes random contractions of individual tissue sections rather than coordinated contraction of the entire mass of tissue of the chamber. The device locks in on the signal, changing signal gain as necessary to track the signal, toward delivering a therapy suitable to return the heart to normal cardiac activity. The filtered and amplified signal is compared with inner and outer targets and the gain is varied according to target crossings.
It is a principal object of the present invention to provide a bradycardia pacemaker which utilizes a switched capacitor amplifier and comparator system to sense abnormal cardiac activity and stimulate the heart accordingly.
Another object of the present invention is to provide highly stable voltage reference levels, using switched capacitor amplification, as targets for comparison with the level of the cardiac signal.
Yet another object of the invention is to provide stable regulation of the supply voltage for the pacemaker, utilizing switched capacitor amplification.